Particulate water-absorbent polymer and production method thereof

Abstract

An object of the present invention is to provide a method for producing a particulate water-absorbent polymer particle having remarkably improved long-term color stability, without an adverse effect (such as a delay in polymerization) on synthesis of the particulate water-absorbent polymer. The method includes the steps of polymerizing a monomer aqueous solution (B) containing (i) at least one type of monomer (A) that is capable of forming a particulate water-absorbent polymer by polymerization, (ii) at least one type of crosslinking agent, (iii) at least one type of polymerization initiator and (iv) an organophosphorus compound in an amount of not less than 1 but not more than 100 ppm by mass with respect to the monomer (A) so as to form a hydrogel polymer, and drying the hydrogel polymer. The method enables to provide an excellent particulate water-absorbent polymer that realizes (i) long-term color stability and improvement in urine tolerance and (ii) absorbing property, which are in such a trade-off relationship. In this way, the above object is attained.

1. A particulate water-absorbent polymer obtained through a method comprising:
 polymerizing a monomer aqueous solution (B) containing (i) at least one of monomer (A) that is capable of forming a water-absorbent polymer by polymerization, (ii) at least one of crosslinking agent, (iii) at least of polymerization initiator and (iv) an organic phosphorous compound in an amount of not less than 1 ppm by mass but not more than 100 ppm by mass with respect to the monomer (A) so as to obtain a hydrogel polymer; and thermally drying the hydrogel polymer; 
 the particulate water-absorbent polymer exhibiting an L value(Lightness) of 70 or higher in Hunter's Lab color system after 7-day exposure to atmosphere of 70±1° C. of temperature and 65±1% of relative humidity and comprising iron in an amount based on Fe 2 O 3  of not less than 0.001 ppm by mass but not more than 5 ppm by mass with respect to the particulate water-absorbent polymer. 
 
     
     
       2. The particulate water-absorbent polymer according to  claim 1 , wherein the monomer solution (B) contains the organic phosphorous compound in amount of not less than 30 ppm by mass but not more than 100 ppm by mass with respect to the monomer (A). 
     
     
       3. The particulate water-absorbent polymer according to  claim 1 , wherein the at least one of monomer (A) contains not less than 70 mol % but not more than 100 mol % of acrylic acid and salt thereof, and the acrylic acid and salt thereof contain not less than 1 mol % but not more than 50 mol % of acrylic acid and not less than 50 mol % but not more than 99 mol % of an alkali metal salt of acrylic acid. 
     
     
       4. The particulate water-absorbent polymer according to  claim 1 , the method comprising surface-crosslinking the particulate water-absorbent polymer. 
     
     
       5. The particulate water-absorbent polymer according to  claim 4 , wherein the step of surface-crosslinking includes heating the particulate water-absorbent polymer at a heating temperature of not less than 150° C. but not more than 250° C. 
     
     
       6. The particulate water-absorbent polymer according to  claim 1 , the method comprising adding the organic phosphorous compound to the hydrogel polymer or to the particulate water-absorbent polymer, after the polymerization. 
     
     
       7. The particulate water-absorbent polymer according to  claim 1 , wherein moisture content of the particulate water-absorbent polymer is not more than 5 mass %. 
     
     
       8. The particulate water-absorbent polymer according to  claim 1 , wherein the hydrogel polymer is thermally dried at a heating temperature of not less than 170° C. but not more than 220° C. 
     
     
       9. The particulate water-absorbent polymer according to  claim 1 , wherein particles having a particle size of 150 to 600 μm is 60 to 100 mass % with respect to a total amount of the particulate water-absorbent polymer. 
     
     
       10. A particulate water-absorbent polymer obtained through a method comprising:
 polymerizing a monomer aqueous solution (B) containing (i) at least one of monomer (A) that is capable of forming a water-absorbent polymer by polymerization, (ii) at least one of crosslinking agent and (iii) at least one of polymerization initiator so as to obtain a hydrogel polymer; and 
 thermally drying the hydrogel polymer, 
 wherein the hydrogel polymer before the drying contains an organic phosphorous compound and iron in an amount based on Fe 2 O of not less than 0.001 ppm by mass but not more than 5 ppm by mass with respect to the particulate water-absorbent polymer, 
 a drying temperature during the step of thermally drying is not less than 150° C. but not more than 250° C., and 
 the particulate water-absorbent polymer contains the organic phosphorous compound in an amount of not less than 30 ppm by mass but not more than 500 ppm by mass with respect to the particulate water-absorbent polymer, the particulate water-absorbent polymer satisfying at least one of the followings (a) to (c): 
 (a) a particle of the particulate water-absorbent polymer after 7-day exposure to atmosphere of 70±1° C. of temperature and 65±1% of relative humidity exhibits an L value (Lightness) of 70 or higher in Hunter's Lab color system; 
 (b) the particulate water-absorbent polymer contains particles having a diameter of less than 150 μm in an amount of 0 or more mass % but not more than 5 mass %; a mass median particle size (D50) is not less than 200 μm but not more than 600 μm; and a logarithmic standard deviation (σζ) of a particle size distribution is not less than 0.20 but not more than 0.40; and 
 (c) an absorbency against pressure (AAP) under pressure of 1.9 kPa or 4.8 kPa for 0.90 mass % sodium chloride aqueous solution for 60 minutes is at least 20 (g/g). 
 
     
     
       11. The particulate water-absorbent polymer according to  claim 10 , wherein the monomer solution (B) contains the organic phosphorous compound in amount of not less than 30 ppm by mass but not more than 100 ppm by mass with respect to the monomer (A). 
     
     
       12. The particulate water-absorbent polymer according to  claim 10 , wherein the at least one of monomer (A) contains not less than 70 mol % but not more than 100 mol % of acrylic acid and salt thereof, and the acrylic acid and salt thereof contain not less than 1 mol % but not more than 50 mol % of acrylic acid and not less than 50 mol % but not more than 99 mol % of an alkali metal salt of acrylic acid. 
     
     
       13. The particulate water-absorbent polymer according to  claim 10 , wherein the monomer aqueous solution (B) contains iron in an amount of not less than 0.001 ppm by mass but not more than 5 ppm by mass (based on Fe 2 O 3 ) with respect to the monomer (A). 
     
     
       14. The particulate water-absorbent polymer according to  claim 10 , the method comprising surface-crosslinking the particulate water-absorbent polymer. 
     
     
       15. The particulate water-absorbent polymer according to  claim 14 , wherein the step of surface-crosslinking includes heating the particulate water-absorbent polymer at a heating temperature of not less than 150° C. but not more than 250° C. 
     
     
       16. The particulate water-absorbent polymer according to  claim 10 , the method comprising adding the organic phosphorous compound to the hydrogel polymer or to the particulate water-absorbent polymer, after the polymerization. 
     
     
       17. The particulate water-absorbent polymer according to  claim 10 , wherein moisture content of the particulate water-absorbent polymer is not more than 5 mass %. 
     
     
       18. The particulate water-absorbent polymer according to  claim 10 , wherein the hydrogel polymer is thermally dried at a heating temperature of not less than 170° C. but not more than 220° C. 
     
     
       19. The particulate water-absorbent polymer according to  claim 10 , wherein particles having a particle size of 150 to 600 μm is 60 to 100 mass % with respect to a total amount of the particulate water-absorbent polymer. 
     
     
       20. A particulate water-absorbent polymer obtained through a method comprising:
 polymerizing a monomer aqueous solution (B) containing (i) at least one of monomer (A) that is capable of forming a water-absorbent polymer by polymerization, (ii) a least one of crosslinking agent, and (iii) at least one of polymerization initiator so as to obtain a hydrogel polymer; and thermally drying the hydrogel polymer, wherein the hydrogel 
 
       polymer before the drying contains an organic phosphorous compound, and a drying temperature during the step of thermally drying is not less than 150° C. but not more than 250° C.,
 the particulate water-absorbent polymer exhibiting an L value (Lightness) of 70 or higher in Hunter's Lab color system after 7-day exposure to atmosphere of 70±1° C. of temperature and 65±1% of relative humidity and the particulate water-absorbent polymer further comprising iron in amount based on Fe 2 O 3  of not less than 0.001 ppm by mass but not more than 5 ppm by mass with respect to the particulate water-absorbent polymer. 
 
     
     
       21. The particulate water-absorbent polymer according to  claim 20 , wherein particles having a particle size of 150 to 600 μm is 60 to 100 mass % with respect to a total amount of the particulate water-absorbent polymer. 
     
     
       22. A particulate water-absorbent polymer containing an organic phosphorous compound in an amount of not less than 30 ppm by mass but not more than 500 ppm by mass with respect to the particulate water-absorbent polymer, and iron in an amount based on Fe 2 O 3  of not less than 0.001 ppm by mass but not more than 5 ppm by mass with respect to the particulate water-absorbent polymer, the particulate water-absorbent polymer satisfying at least one of the followings (a) to (c):
 (a) a particle of the particulate water-absorbent polymer after 7-day exposure to atmosphere of 70±1° C. of temperature and 65±1% of relative humidity exhibits an L value (Lightness) of 70 or higher in Hunter's Lab color system; 
 (b) the particulate water-absorbent polymer contains particles having a diameter of less than 150 μm in an amount of 0 or more mass % but not more than 5 mass %; a mass median particle size (D50) is not less than 200 μm but not more than 600 μm; and a logarithmic standard deviation (σζ) of a particle size distribution is not less than 0.20 but not more than 0.40; and 
 (c) an absorbency against pressure (AAP) under pressure of 1.9 kPa or 4.8 kPa for 0.90 mass % sodium chloride aqueous solution for 60 minutes is at least 20 (g/g). 
 
     
     
       23. The particulate water-absorbent polymer according to  claim 22 , wherein moisture content of the particulate water-absorbent polymer is not more than 5 mass %. 
     
     
       24. The particulate water-absorbent polymer according to  claim 22 , wherein particles having a particle size of 150 to 600 μm is 60 to 100 mass % with respect to a total amount of the particulate water-absorbent polymer.